1. Field of the Invention
This invention relates to a manufacturing method of thin film transistors used in a liquid crystal display, a line sensor or the like.
2. Description of Related Art
Vigorous research and development has been made for semiconductor devices using polycrystalline silicon since polycrystalline silicon represents the largest carrier mobility among various kinds of thin film semiconductors. A representative semiconductor device of the type referred to above is a thin film transistor (referred to as a TFT hereinafter) used as a switching element for picture cell of a liquid crystal display, a driving circuit for a line sensor, etc. A variety of manufacturing methods of the TFTs have been proposed until now (IEEE ELECTRON DEVICE LETTERS. VOL. 9, NO. 6, JUNE 1988 pp290-292, etc.).
A conventional manufacturing method of TFTs using polycrystalline silicon will be discussed hereinbelow with reference to FIG. 1. A polycrystalline silicon film 32 is formed directly on an insulative substrate 31 made of silica by the low-pressure chemical vapor deposition (LPCVD) method (FIG. 1(a)). The temperature of the substrate at this time is not lower than 620.degree. C. A silicon oxide film or silicon nitride film is formed as a gate insulative film 33 over the polycrystalline silicon film 32 and substrate 31, onto which a gate electrode 34 is patterned (FIG. 1(b)). Phosphorous is implanted from a direction of an arrow by ion implantation method so as to form a conductive polycrystalline silicon film for obtaining ohmic contact between a drain electrode and a source electrode which will be formed later and the polycrystalline silicon film 32. The implanted phosphorous is activated by annealing at 1000.degree. C. or more. As a result, the polycrystalline silicon film 32 not covered with the gate electrode 34 is changed to an n-type polycrystalline silicon film 35 (FIG. 1(c)). The gate insulative film 33 in the region corresponding to the drain electrode and source electrode is removed by etching, thereby forming contact holes 36 (FIG. 1(d)). After the drain electrode 37 and source electrode 38 are formed, the TFT is completed (FIG. 1(e)).
The above-described manufacturing method utilizes a direct forming method wherein a polycrystalline silicon film is formed directly by the LPCVD method. Different from the direct forming method, a recrystallization method is also well known, wherein an amorphous silicon film is formed by plasma enhanced chemical vapor deposition (PCVD) method and is annealed by heat or laser beams thereby enabling recrystallization. However, the aforementioned methods for forming the polycrystalline silicon film have the following drawbacks.
Since a high temperature not lower than 600.degree. C. is required in the direct forming method, a substrate to be used should have favorable heat resistance as silica or the like, causing an increase of manufacturing costs. The recrystallization method with heat (solid phase crystallization method) has a similar problem as outlined above. On the other hand, according to the recrystallization method with laser beams (laser recrystallization method), although an expensive substrate is not necessary, it is generally difficult to make the quality of the film, e.g., hydrogen content in the original amorphous silicon film optimal, and moreover, good reproducibility cannot be expected because of the insufficient stability of outputs of laser beams at present.
Further, in the conventional manufacturing method of TFTs described above, the conductive polycrystalline silicon film for ohmic contact should be formed at a high temperature not lower than 1000.degree. C., requiring a costly heat-proof substrate such as a silica substrate. Further, the ion implantation method necessitates an accelerating mechanism to implant a dopant, making it difficult to enlarge the area of the substrate.
In the meantime, another kind of TFT is present which has a semiconductive thin film at a channel part made of non-crystalline silicon and a semiconductive thin film in contact with source and drain electrodes made of polycrystalline silicon. The non-crystalline silicon referred to here is a general term of amorphous silicon and microcrystalline silicon. In order to manufacture the TFT in the aforementioned structure, the amorphous silicon film is generally formed by the plasma gas decomposition method, CVD method, sputtering method, electron cyclotron resonance (ECR) method or the like. The temperature of the substrate is set to be not higher than 500.degree. C. in any of the above methods. On the other hand, in forming the polycrystalline silicon film, the direct forming method, solid phase crystallization method, or laser recrystallization method, etc. as described earlier is used. The polycrystalline silicon film should be annealed at a temperature higher than the forming temperature of the amorphous silicon film in the direct forming method and solid phase recrystallization method, and therefore it is impossible to form both the amorphous silicon film and polycrystalline silicon film on the same substrate. In contrast, although both films may be formed on the same substrate according to the laser-recrystallization method, since the annealing process is conducted locally by laser beams, the stability of laser outputs is low, resulting in poor reproducibility.
Meanwhile, Japanese Patent Application Laid-Open No. 63-185015 (185015/1988) discloses a different method to form a polycrystalline silicon film on the substrate. According to the method disclosed therein, after an amorphous silicon film is formed on an insulative substrate, a dopant is implanted into the surface area of the amorphous silicon film and thermally treated. Thereafter, the layer where the dopant is included is removed. This method also requires implantation of the dopant, thus making it difficult to increase the area of the substrate.